Although part of the life history of loggerhead turtles (Caretta caretta) in the northwest Pacific population has been documented, there is still a paucity of data regarding their spatiotemporal ecology at the local level. The Sanriku Coast is known to be the high-latitude foraging grounds (39° N) for loggerhead turtles of the northwest Pacific population. This site is located far north from the northernmost loggerhead rookeries (>400 km) and more than 10 years of surveys have found that loggerhead turtles migrate there only in the summer. To better understand the spatiotemporal ecology of this population along the Sanriku Coast, the present study investigated the sex of 164 loggerhead turtles captured as bycatch along the Sanriku Coast between 2013 and 2017 by measuring the external morphology and serum testosterone concentrations. These analyses suggested that most of the adult-sized turtles in the Sanriku foraging grounds were males (few females), whereas both sexes of immature-sized turtles migrated to this Coast. This is possibly because migration to the Sanriku Coast is restricted to summer due to the seasonal sea surface temperature shift, and this period overlaps with the nesting period. Therefore, only immature turtles and adult males may be able to migrate. Understanding sex differences in spatiotemporal ecology should contribute to conservation practice in this population, where global warming, together with the turtle's temperature-dependent sex determination, can be expected to reduce the proportion of male hatchlings in the future.